Power control system and method

ABSTRACT

A system and method for controlling an electrical device is provided. The method comprises receiving three phase power from a source, decomposing signals representative of power in each phase of the three phase power to provide a positive-sequence component of each phase and tracking the positive-sequence component of each phase via a phase locked loop and a tracking filter.

BACKGROUND

The invention relates to power systems and more specifically to a systemand method for controlling power.

Electric sources may be connected or organized in a network to enablethe transmission of power to various devices, or communication betweenthe devices. Such a network of interconnected devices may be describedas a grid. For example, a power grid may be an interconnected networkfor delivering electricity from one or more power generators to theconnected devices (e.g., customers of the utility company). A power gridmay transmit AC power at a frequency, amplitude, and/or phase angle to alarge number of electrical devices. Synchronized operation of a grid, orportions of a grid, may enable a pooling of power generation, as well asa pooling of loads to result in lower operating costs.

Though transmitting synchronized AC power may be beneficial to theefficient transmission and/or distribution of power, many factors maydisturb the synchronization of a grid. For example, a power gridincluding an unbalanced power source may disturb the synchronization ofthe grid.

Typically, phase lock loops (PLLs) are used in power systems forsynchronization. PLLs under balanced conditions generate a signal thathas a fixed relation to the phase of an input signal. PLLs respond toboth the frequency and the phase of the input signals, automaticallyraising or lowering the frequency of a controlled oscillator until it ismatched to the reference in both frequency and phase. However, PLLs havelimited success when employed in grids having unbalanced power sources.

Therefore there is a need to implement a control system thatsynchronizes the input power signals to the grid output signals.

BRIEF DESCRIPTION

Briefly, according to one embodiment of the invention, a method forcontrolling an electrical device is provided. The method comprisesreceiving three phase power from a source, decomposing signalsrepresentative of power in each phase of the three phase power toprovide a positive-sequence component of each phase, and tracking thepositive-sequence component of each phase via a phase locked loop and atracking filter.

In another embodiment, a method for controlling an electrical device isprovided. The method comprises receiving three phase power from asource, applying the three phase power to a tracking filter to generatebalanced quadrature signals representative of each phase of power,decomposing the quadrature signals to obtain a positive-sequencecomponent of each phase of power, applying the positive-sequencecomponents to a phase locked loop to generate signals for each phaserepresentative of rotational frequency or speed, and applying thesignals representative of rotational frequency or speed to the trackingfilter to track the positive-sequence components.

In another embodiment, a system for controlling an electrical device isprovided. The system comprises a tracking filter device configured todetermine quadrature signals representative of each phase of three phasepower from a source, and a decomposition circuit configured to decomposethe quadrature signals to obtain a positive-sequence component of eachphase of power. The system further includes a phase locked loop coupledto receive the positive-sequence components and configured to generatesignals for each phase representative of rotational frequency or speed,the signals representative of rotational frequency or speed beingapplied to the tracking filter to track the positive-sequencecomponents.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a block diagram of an embodiment of a power control systemimplemented according to one aspect of the invention;

FIG. 2 is a block diagram illustrating the components of a trackingfilter device used in a power system according to one embodiment of theinvention;

FIG. 3 is a block diagram of another embodiment of a power controlsystem implemented according to an aspect of the invention;

FIG. 4 is a graph illustrating current unbalance in an power gridcurrent having three phases;

FIG. 5 is a graph illustrating positive-sequence components derived fromunbalance input current;

FIG. 6 is a graph depicting the effect of applying the presenttechniques to synchronize the grid having unbalanced power. and

FIG. 7 is a flow chart illustrating one method by which an electricaldevice is controlled.

DETAILED DESCRIPTION

Turning now to the drawings, and referring first to FIG. 1, a powersystem is illustrated. Power system 10 comprises a power grid 12, atracking filter device 14, a decomposition network 16 and a phase lockedloop (PLL) 18. Each block is described in further detail below.

Power grid 10 is configured to provide three phase power to variouselectrical loads (not shown). Power generated and distributed by varioussources (e.g., a power plant, a generator, etc.) may be synchronized infrequency, amplitude, and/or phase angle. Synchronization of AC powerresults in the efficient transmission and/or distribution of power.Power from the grid may be applied to the downstream circuitry discussedbelow by any suitable components, such as transformers, power lines,in-plant power distribution equipment and buses, and so forth. Moreover,the grid-side components may include certain upstream switchgear andprotection devices, such as disconnects, fuses, and so forth (notseparately represented).

Tracking filter device 14 is configured to receive each phase of threephase power from the power grid and to convert the signals into balancedquadrature signals. In one embodiment, the tracking filter device 14includes three tracking filters configured for trackingpositive-sequence components of each of the phase. The tracking filterdevice itself may be of a type described in U.S. patent application Ser.No. 12/627,400, entitled “Digital Implementation of a Tracking Filter”,and filed on Nov. 30, 2009 and Ser. No. 12/627,472, entitled “A PhaseLocked Loop with Tracking Filter for Synchronizing an Electric Grid”,filed on Nov. 30, 2009 both of which are incorporated herein in theirentirety.

Decomposition circuit 16 is configured to decompose the balancedquadrature signals to symmetric components. The symmetric componentsinclude a positive-sequence component and a negative component of eachphase of power. In one embodiment, the positive-sequence components ofthe phases are substantially similar but displaced by 120 degrees fromone another.

PLL 18 receives the positive-sequence components generated by thedecomposition circuit 16. PLL 18 is configured to generate signals foreach phase representative of rotational frequency or speed (therotational speed being a function of the electrical frequency). Thesignals representative of rotational frequency or speed is applied tothe tracking filter to track the positive-sequence component of theinput signals. In one embodiment, the PLL is synchronized with apositive-sequence of the source. The manner in which the symmetriccomponents are generated is described in further detail below.

FIG. 2 is a block diagram illustrating the components of a trackingfilter device used in a power system according to one embodiment of theinvention. The tracking filter device includes three tracking filters20, 22 and 24 respectively. Each block is described in further detailbelow.

The tracking filter device is configured to receive each phase of threephase power generated by an AC power source in the power grid 12.Specifically, tracking filter 20 is configured to receive “phase A” ofthe three phase power and is represented by reference numeral 30.Similarly, tracking filter 22 is configured to receive “phase B” of thethree phase power and is represented by reference numeral 32, andtracking filter 24 is configured to receiving “phase C” of the threephase power and is represented by reference numeral 34.

Tracking filter 20 is configured to generate balanced quadrature signalsrepresented by reference numerals 38 and 40. Similarly, tracking filter22 is configured to generate balanced signals represented by referencenumerals 42 and 44, and tracking filter 24 is configured to generatebalanced signals represented by reference numerals 46 and 48. In oneembodiment, the tracking filters are non-linear. The quadrature signalsx₁ and x₂ generated by tracking filter 20 (and/or tracking filters 22and 24) may be represented by the following state-space equation:

$\begin{matrix}{\begin{bmatrix}x_{1}^{\prime} \\x_{2}^{\prime}\end{bmatrix} = {{\begin{bmatrix}{- a} & {- \omega_{0}} \\\omega_{0} & 0\end{bmatrix}*\begin{bmatrix}x_{1} \\x_{2}\end{bmatrix}} + {\begin{bmatrix}a \\0\end{bmatrix}*u}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where “ω₀” represents an angular frequency of the input signal and“a”=is a constant parameter associated to band-width.

The balanced quadrature signals generated by tracking filters 20, 22 and24 are applied to decomposition circuit 16. The decomposition circuit 16decomposes each set of quadrature signals into correspondingpositive-sequence components and negative-sequence components. Thepositive-sequence components of the three phases f_(a1), f_(b1) andf_(c1) may be represented by the following equation:

$\begin{matrix}{\begin{bmatrix}f_{a\; 1} \\f_{b\; 1} \\f_{c\; 1}\end{bmatrix} = {\frac{1}{3}{\left\{ {\begin{bmatrix}1 & {- \frac{1}{2}} & {- \frac{1}{2}} \\{- \frac{1}{2}} & 1 & {- \frac{1}{2}} \\{- \frac{1}{2}} & {- \frac{1}{2}} & 1\end{bmatrix} + {j\begin{bmatrix}0 & \frac{\sqrt{3}}{2} & {- \frac{\sqrt{3}}{2}} \\{- \frac{\sqrt{3}}{2}} & 0 & \frac{\sqrt{3}}{2} \\\frac{\sqrt{3}}{2} & {- \frac{\sqrt{3}}{2}} & 0\end{bmatrix}}} \right\} \begin{bmatrix}f_{a} \\f_{b} \\f_{c}\end{bmatrix}}}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

where f_(a), f_(b) and f_(c) can be current, voltage, flux linkage orsimilar of the signals in phases A, B and C respectively. The outputsignal 26 of the decomposition circuit 16 is provided to PLL 18. PLL 18is configured to generate signals for each tracking filterrepresentative of rotational frequency or speed. The signals 36representative of electrical frequency or electrical angular speed“ω_(e)” is applied to each tracking filter 20, 22 and 24 as input ω₀ totrack the positive-sequence components. Thus, even if a voltageunbalance exists in a power grid, the PLL is synchronized with apositive-sequence component set of the power signals.

In another embodiment of the power control system, PLL 18 is used tocompute the phase angle “θ_(e)” of the positive sequence components.FIG. 3 is a block diagram of another embodiment of power control system58 including a first PLL 18 and a second PLL 28. PLL 18 is used togenerate signals representative of the phase angle “θ_(e)” of thepositive sequence components. The output signal 27 of PLL 18 is providedto a load (for example, a power converter) coupled to the power controlsystem.

PLL 28 receives three phase power from power grid 12 and is configuredto generate signals “ω_(e)” for each tracking filter representative ofrotational frequency or speed. PLL 28 is similar to the phase lockedloop device disclosed in U.S. patent application Ser. No. 12/527,472,entitled “A phase locked loop with tracking filter for synchronizing anelectric grid”, filed on 30 Dec. 2009. The embodiment discussed abovehas a substantially improved dynamic response. FIG. 4 is a graphdepicting current unbalance in a power grid current having three phases.Graph 58 depicts unbalanced input currents 60, 62 and 64 in phase A,phase B and phase C respectively. It may be noted that the unbalancedcurrents may behave similarly to unbalanced voltages. The graph 58 showsthe three-phases of the current vectors in the time domain.

FIG. 5 is a graph depicting symmetric components generated by thedecomposition circuit. Graph 66 depicts symmetric components 68, 70 and72 synchronized with a positive-sequence set of the current from thesource in phase A, B and C respectively. The PLL is configured tosynchronize the symmetric components generated by the decompositioncircuit with the positive-sequence set of the input current.

FIG. 6 is a graph depicting the effect of applying the presenttechniques to synchronize the grid having unbalanced power. The graph 74depicts the output angular frequency 76 of a PLL over time 80. As seenin the graph 74, the output estimate 78 may fluctuate substantially inthe portion of the graph marked “disabled,” which is approximately from0 to 2.36 seconds. Once a PLL using a decomposition network is enabled,the output estimate 78 fluctuate less, as shown in the portion of thegraph marked “enabled,” which is approximately from 2.36 to 2.75seconds. The remaining fluctuations may result from other harmonicswhich are outside the bandwidth of the tracking filter.

FIG. 7 is a flow chart illustrating one method by an electrical systemis controlled. At step 84, a three phase power from a source. The threephase power may be supplied to various electrical devices using a powergrid.

At step 86, the three phase power is applied to a tracking filter togenerate balanced quadrature signals representative of each phase ofpower. In one embodiment, the balanced quadrature signals are generatedvia a separate tracking filter for each of the three phases.

At step 88, the balanced quadrature signals are decomposed to obtainsymmetric components for each phase. The symmetric components includepositive-sequence components and negative-sequence components. In oneembodiment, the positive-sequence components of the phases aresubstantially similar but displaced by 120 degrees from one another. Ina further embodiment, the balanced quadrature signals are decomposed toobtain a negative component for each phase of power.

At step 90, the positive-sequence components are applied to a phaselocked loop to generate signals for each phase representative ofrotational frequency or speed. The signals representative of rotationalfrequency or speed are then applied to the tracking filter to track thepositive-sequence components. In one embodiment, the PLL is synchronizedwith a positive-sequence set of the source.

The above described techniques have many advantages including providingsynchronization to power grid for standalone system drives anddistributed generation sources. The technique provides a high dynamicsolution to substantially reduce ‘dirty’ grid synchronization problems.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1.-20. (canceled)
 21. A method for controlling an electrical device,comprising: receiving three phase power from a source; decomposingsignals representative of power in each phase of the three phase powerto provide a separate sequence component of each phase; and tracking theseparate sequence component of each phase via a phase locked loop and atracking filter.
 22. The method of claim 21, wherein the trackingfilters synchronize the phase locked loop to the power phases via thetracking filter and the sequence components despite phase imbalancesbetween the power phases.
 23. The method of claim 21, wherein theseparate sequence components of the phases are substantially similar butdisplaced by 120 degrees from one another.
 24. The method of claim 21,wherein the phase locked loop provides a rotational frequency or speedsignal to the tracking filter.
 25. The method of claim 24, wherein thetracking filter comprises filters for each of the three power phases,and wherein each filter receives a rotational frequency or speed signalfrom the phase locked loop, and a signal from a respective power phase.26. The method of claim 21, wherein decomposing signals representativeof power in each phase comprises generating balanced quadrature signals.27. The method of claim 21, further comprising decomposing signalsrepresentative of power in each phase of the three phase power toprovide positive and negative components of each phase.
 28. The methodof claim 21, wherein the phase locked loop provides a phase anglesignal.
 29. A method for controlling an electrical device, comprising:receiving three phase power from a source; applying the three phasepower to a tracking filter to generate balanced quadrature signalsrepresentative of each phase of power; decomposing the quadraturesignals to obtain a separate sequence component of each phase of power;applying the positive-sequence components to a phase locked loop togenerate signals for each phase representative of rotational frequencyor speed; and applying the signals representative of rotationalfrequency or speed to the tracking filter to track the separate sequencecomponents.
 30. The method of claim 29, wherein the separate sequencecomponents are tracked via a separate tracking filter for each of thephases.
 31. The method of claim 29, wherein the three phase power isreceived from an imbalanced power source.
 32. The method of claim 31,wherein the tracking filter synchronizes the phase locked loop to thepower phases despite phase imbalances between the power phases.
 33. Themethod of claim 29, wherein the quadrature signals are decomposed toobtain positive and negative components of each phase of power.
 34. Themethod of claim 29, wherein the separate sequence components of thephases are substantially similar but displaced by 120 degrees from oneanother.
 35. A system for controlling an electrical device, comprising:tracking filter configured to determine quadrature signalsrepresentative of each phase of three phase power from a source; adecomposition circuit configured to decompose the quadrature signals toobtain a separate sequence component of each phase of power; and a phaselocked loop coupled to receive the sequence components and configured togenerate signals for each phase representative of rotational frequencyor speed, the signals representative of rotational frequency or speedbeing applied to the tracking filter to track the sequence components.36. The system of claim 35, wherein the separate sequence components ofthe phases are substantially similar but displaced by 120 degrees fromone another.
 37. The system of claim 35, wherein the tracking filtercomprises filters for each of the three power phases, and wherein eachfilter receives a rotational frequency or speed signal from the phaselocked loop, and a signal from a respective power phase.
 38. The systemof claim 35, wherein the tracking filter device comprises three trackingfilters configured for tracking positive-sequence components of each ofthe phase.
 39. The system of claim 35, wherein the tracking filtersynchronizes the phase locked loop to the power phases via the trackingfilter and the sequence components despite phase imbalances between thepower phases.
 40. The system of claim 35, wherein the decompositioncircuit is configured to decompose the quadrature signals to obtainpositive and negative components of each phase of power.